Dynamic data storage and retrieval has become of very great importance in our increasingly information based society. In both our work and enjoyment we typically use computers or computerized systems which read and write data on various storage media contained in removable or installed ("fixed") storage units. Users of such storage systems typically want to handle a lot of data both efficiently and safely, and at low cost. Today a ubiquitous example of a storage unit generally meeting these criteria is the hard disk drive (hereinafter "hard drive"). Worldwide some 200,000 hard drives are manufactured every day.
Hard drives consist of one or more spindle mounted disks which have magnetic media on one or, more typically, both major sides. The terms "disk pack" and "disk platter" (or even simply "platter") are widely used terms for an assembly of such disks. A motor is provided to rotate the disk assembly and arms bearing read/write heads ("R/W head") are positioned pivotally or linearly above the media to magnetically write or read data into and from the media (an assembly of such arms and R/W heads is commonly termed a "head stack"). To efficiently and reliably later access user data a scheme of concentric tracks and sectors within those tracks are defined during hard drive manufacturing using a process called "servo track writing." This process places servo data in a proprietary coding in the disk media called a "servo pattern." Data storage density in hard drives thus very much depends upon how densely such tracks and sectors can be defined and reliably used. Hard drives coming available today have track densities as high as 10,000 tracks per inch (TPI), and manufactures hope to obtain 20,000 TPI in the next 3-5 years.
Accurately manufacturing hard drives economically and in large quantity is not easy. For example, due to the limitations of mechanical tolerances inherent to manufacturing, the actual speed of rotation of disk platters is not exactly the same in every unit produced. If a hard drive is designed to optimally store n sectors of data in each track, platter revolution speeds that cause n-2 or n+3/4 sectors to be written can cause unexpected and even disastrous results (often at some later point outside of the closely controlled manufacturing environment). Such variation can be termed "sector-inconsistency error." Of course, the hard drive can be designed with tolerances to accommodate an expected degree of sector-inconsistency error, but that seriously undercuts the goal of achieving a high data-per-track storage density.
Further, similarly due to manufacturing limitations, the disk platter rotation is never perfectly circular. If this imperfection is severe enough it can even cause the coding to be mistakenly written to a different data zone, called an "off-track error." One solution for this is to allow the physical width of the track to be such that the possibility of off-track error becomes negligible, but this reduces the TPI and seriously undercuts the goal of achieving a high data-per-platter storage density.
To address these problems, and to a lessor extent others as well, the industry has turned to putting clock information into hard drives prior to writing the servo information. FIG. 1 (background art) is a simplified depiction of this (as noted above, actual hard drives are typically much more complex than this, but this simplification illustrates the necessary principles of operation). Within a workpiece hard drive 1 (shown only in pertinent detail) a media disk 2 rotates on a hub 3. A clock arm 4 bearing a clock head 5 is then introduced and a clock pattern, i.e., a clock track 6, is written at the outer periphery of the media disk 2. Once the clock track 6 is written, the clock head 5 is used to read it back and the regular R/W head 7 of the hard drive 1 is used in a synchronized manner to write the desired servo pattern into the media disk 2.
A servo pattern may also be complex. It may be embedded throughout the data storage area on the media disks or placed on a single media surface dedicated to it (a servo pattern is intentionally not shown in any of the drawings herein because of the confusion which doing so might cause; also, an older wedge servo system has been used in hard drives but is now obsolete). However, it should be noted that in a hard drive only one clock track is needed.
The process of writing the clock pattern and the servo pattern is quite complex, and requires extremely precise timing, measurement, and positioning. First the clock pattern must be written. The platter of media disks is brought up to its operating speed, which commonly will be 5,400, 7,200, or even 10,000 rpm, and an initial clock pattern is written using the clock head. However, this initial pattern usually has one clock increment which is less than or greater than the others (e.g., n-1/2 or n+3/4), and in extreme cases the number of clock increments may even be less than or greater than desired (e.g., n-1 or n+2). Therefore, to create consistent clock information, the initial clock pattern is read back as a measurement of actual hard drive conditions, calculations are performed to determine what is needed to obtain a clock pattern with the desired number of consistent increments, and based upon this a final clock pattern is written.
Next, the media disks are maintained at operating speed and the clock head, which is still introduced to the hard drive, is used to read back the clock pattern while the R/W heads are used to write the servo pattern. Feedback from the clock pattern is used to write the servo pattern in a manner such that data will later be stored in a desired and consistent number of sectors. Concurrently, this feedback from the clock pattern is also used to insure that servo track writing accurately follows the rotation of the disk and that the R/W heads are adjusted to write the servo pattern in a more perfect circle. During this process feedback techniques are used to measure and position the actual R/W heads very accurately during the actual servo pattern writing. Today, laser interferometry generally is used for this but some manufacturers also employ optical encoders.
Unfortunately, there are a number of problems, compromises and lost efficiencies associated with the above-described use of a magnetic disk based clock pattern. It should be appreciated that the clock head and clock pattern are used only in hard drive assembly. Obviously, if the area used by the clock pattern, i.e., the clock track, could otherwise be used for data storage this would increase the storage capacity of the hard drive. Further, because the clock head and the associated electronics for it are expensive and cumbersome, hard drive manufacturers understandably prefer to make these part of the external manufacturing apparatus, rather than include instances of them in every hard drive being produced. But this means that the clock head must specially be introduced into the hard drive during assembly, slowing the assembly process, and perhaps more importantly putting tooling and product in harms way (those familiar with the art of magnetic recording will readily appreciate that for the clock head to write and read the media surface it must be brought very close, typically on the order of 6 micro-inches).
However, to the inventor, based upon his own years as a provider of equipment to many of the largest hard drive manufacturers, the biggest cause for concern is the level of cleanliness which must be maintained during assembly of many storage units using current techniques. As touched upon above, head to media clearances are very small and the relative speed between these when in operation is extremely high. Any contamination that enters the confines of such a storage unit can have catastrophic consequences, which hopefully will turn up early before the manufacturer performs final tests on the unit, but which all to often instead turn up later and cause the loss of an ultimate user's precious data.
Among computer users the term "head crash" is all too well known, it means the catastrophic loss of data by damage to the media surface or even loss of the entire hard drive due to the disk platter and head stack literally jamming. The present inventor frequently has removed part of a hard drive housing to display aspects of working hard drive operation. It has been his observation that in a few hours, maybe a day, some media degradation occurs, as evidenced by the drive having seek problems. Then within a day, or typically at most a week, the hard drive completely jams up. To additionally understand this phenomena in hard drives it should be appreciated that static charged particles are readily attracted to and build up on magnetically charged media surfaces.
Hard drive manufacturers also dread head crashes, because dissatisfied customers often will never buy a product from them again. (This is not at all an exaggeration. Among repair personnel and computers users prejudices are easily built from a few bad experiences, and then often vehemently verbalized to others, which undercuts the market goodwill of the often coincidental particular manufacture.)
To prevent contamination, manufactures resort to using clean rooms for assembly. In hard drive manufacturing the current standard for cleanliness is "Class 10," and the cost of a clean room to achieve this is very high. Based upon the inventor's own experience and extensive conversations with experts in the field, this exceeds U.S. $10,000,000 per hard drive manufacturer today. Further, even with class 10 clean room facilities, rigorous and tedious process control is needed to ensure that no contamination is introduced.
Work has been done in the industry to try and eliminate the need for such cleanliness and the need for clean rooms, but until the present invention this has not become possible. For example, the present inventor is the inventor also of U.S. Pat. No. 5,315,372 for a "Non-Contact Servo Track Writing Apparatus Having Read/Head Arm And Reference Arm," and a number of currently pending patent applications for non-invasive servo pattern writing. Using such non-invasive techniques for servo pattern writing has reduced but not eliminated the need to have storage units open during assembly, because there must still be at least one open portal for the clock head access during clock pattern writing.
Accordingly, what is needed for manufacturing of some important classes of information storage units today are apparatus and methods to better write clock patterns. Such solutions should very preferably work on sealed storage units; and it is highly desirable that they not introduce any new instances of, or even better still, reduce or eliminate existing problems, compromises, or lost efficiencies related to the manufacturer of such information storage units.